1. Field of the Invention
The invention relates generally to a method of forming patterns, and more particularly, to a method of forming patterns improved so as to obtain satisfactory pattern shapes of high resolution and high sensitivity.
2. Description of the Background Art
At present, large scale integrated circuits (LSI) represented by 1M or 4M dynamic random access memories (DRAM) are manufactured by selectively irradiating a positive type photo resist constituted by novolak and naphthoquinone diazide with g-line light (wavelength 436 nm) of a mercury lamps, to be followed by pattern formation. The minimum pattern dimension is 1 .mu.m-0.8 .mu.m. However, the method of forming patterns of half micron will become necessary in accordance with the increase in the integration level of LSIs, as seen in 16MDRAMs. For this purpose, a study has been made using KrF excimer laser as a light source having a short wavelength in the method of forming patterns.
As a resist used for deep UV light represented by a KrF excimer laser, a positive type photo resist of novolak-naphthoquinone diazide type such as PR1024 (product of MacDERMID INC.), polymethyl methacrylate (PMMA), polyglycidyl methacrylate (PGMA), polychloromethylated styrene (CMS), etc. are proposed. PMMA and PGMA have low sensitivity and low dry etching resistance. CMS has satisfactory dry etching resistance, but the sensitivity thereof is low. The dry etching resistance of PR1024 is satisfactory, and the sensitivity thereof is high in comparison with the above resist. However, its sensitivity is low when compared with exposure by g-line.
A conventional method of forming patterns will be described hereinafter.
FIGS. 6A-6C show the sectional views of a positive type resist of novolak-naphthoquinone diazide type (PR1024, for example) under a conventional method of forming patterns.
Referring to FIG. 6A, PR1024 is applied on a substrate 2 and prebaked to obtain a resist film of 1.0 .mu.m film thickness.
Referring to FIG. 6B, a KrF excimer laser 4 selectively irradiates resist film 1 with a mask 5. This divides resist film 1 into irradiation regions 1a and non-irradiation regions 1b.
Referring to FIG. 6C, development is carried out using tetra methyl ammonium hydroxide aqueous solution of 2.38 wt. % to obtain resist patterns 9 with irradiation regions 1a removed.
The conventional method of forming patterns explained above has the following problems.
Because the absorption of deep UV light is high in novolak-naphthoquinone diazide type positive resists such as PR1024, light absorption at the surface of resist film 1 is so high that the light will not reach the lower layer portion of resist film 1, as in FIG. 6B. As a result, the sectional shape of resist patterns 9 is tapered upwards to become a triangular shape after development, as in FIG. 6C, leading to a problem that fine patterns could not be obtained precisely.
In a conventional method of forming patterns using not deep UV light but light with a wavelength of 300-500 nm where a step 2a exists in substrate 2, light 20 was scattered by step 2a so that a satisfactory pattern shape could not be obtained (called the notching phenomenon). Similarly, in the case where a film likely to reflect light, such as Al, was formed on substrate 2, satisfactory pattern shapes could not be obtained because of the effect of the reflection of light.
FIGS. 8A-8D show another conventional example of a method of forming resist patterns, which is described in Japanese Patent Laying-Open No. 63-253356.
Referring to FIG. 8A, a resist film 1 of novolak-naphthoquinone diazide type is formed on substrate 2. Next, using mask 5, light having a wavelength of 300-400 nm from high pressure mercury or the like selectively irradiates resist film 1. Crosslinking reaction of the resin occurs at irradiation region 1a by this irradiation of light.
Referring to FIG. 8B, the photo sensitive agent is completely decomposed by irradiating resist film 1 with a light of the same wavelength.
Referring to FIG. 8C, trimethylsilyl dymethyl amine vapor acts on the entire surface of substrate 2. This selectively silylates the portion excluding irradiation regions 1a, that is, the upper layer portion of non-irradiation region 1b, to be converted to a silylated layer 8.
By development of reactive ion etching (RIE) using O.sub.2 gas, silylated layer 8 remains as a SiO.sub.2 layer 13, whereas irradiation region 1a is removed. This forms resist pattern 9 on substrate 2, as shown in FIG. 8D.
In accordance with the aforementioned other conventional example, it can be seen from FIG. 8C that the selectivity of silylation reaction (the ratio of silylation reaction in the upper layer portion of irradiation region 1a to the silylation reaction in the upper portion of non-irradiation region 1b) is low. This results in the division between the exposed portion and the non-exposed portion to be not clear, as in FIG. 8D, leading to a problem that fine patterns could not be obtained precisely. A light having a wavelength of 300-400 nm was used in this method. Because this light has high transmittance, the problem of notching effect described associated with FIG. 7 exists.